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2.4The Principle of Inertia
Physical effects relate only to a change in velocity
Consider two statements that were at one time made with the utmost
seriousness:
People like Galileo and Copernicus who say the earth is rotating must be
crazy. We know the earth can’t be moving. Why, if the earth was really
turning once every day, then our whole city would have to be moving
hundreds of leagues in an hour. That’s impossible! Buildings would shake
on their foundations. Gale-force winds would knock us over. Trees would fall
down. The Mediterranean would come sweeping across the east coasts of
Spain and Italy. And furthermore, what force would be making the world
turn.
All this talk of passenger trains moving at forty miles an hour is sheer
hogwash! At that speed, the air in a passenger compartment would all be
forced against the back wall. People in the front of the car would suffocate,
and people at the back would die because in such concentrated air, they
wouldn’t be able to expel a breath.
Some of the effects predicted in the first quote are clearly just based on
a lack of experience with rapid motion that is smooth and free of vibration.
But there is a deeper principle involved. In each case, the speaker is assum-
ing that the mere fact of motion must have dramatic physical effects. More
subtly, they also believe that a force is needed to keep an object in motion:
the first person thinks a force would be needed to maintain the earth’s
rotation, and the second apparently thinks of the rear wall as pushing on
the air to keep it moving.
Common modern knowledge and experience tell us that these people’s
predictions must have somehow been based on incorrect reasoning, but it is
not immediately obvious where the fundamental flaw lies. It’s one of those
things a four-year-old could infuriate you by demanding a clear explanation
of. One way of getting at the fundamental principle involved is to consider
how the modern concept of the universe differs from the popular concep-
tion at the time of the Italian Renaissance. To us, the word “earth” implies a
planet, one of the nine planets of our solar system, a small ball of rock and
dirt that is of no significance to anyone in the universe except for members
of our species, who happen to live on it. To Galileo’s contemporaries,
however, the earth was the biggest, most solid, most important thing in all
of creation, not to be compared with the wandering lights in the sky known
as planets. To us, the earth is just another object, and when we talk loosely
about “how fast” an object such as a car “is going,” we really mean the car-
object’s velocity relative to the earth-object.
Motion is relative
According to our modern world-view, it really isn’t that reasonable to
expect that a special force should be required to make the air in the train
have a certain velocity relative to our planet. After all, the “moving” air in
the “moving” train might just happen to have zero velocity relative to some
other planet we don’t even know about. Aristotle claimed that things
“naturally” wanted to be at rest, lying on the surface of the earth. But
experiment after experiment has shown that there is really nothing so
Chapter 2Velocity and Relative Motion